Hmph, so it turns out he let her escape on purpose. Admittedly, that wasn't half bad of an idea because it could've led them to another bandit hideout, based on a possible hunch that she was a member of the bandits and the chance that some might have survived or did not join the ambush that night.
I am however, not too keen on her easily joining them even for "atonement" purposes. This is quickly veering off into harem territory with more and more girls informally joining the party. I guess she could be his next disciple or something now that Rina has technically graduated from his mentorship a few years before.